Laser processing with the use of infrared laser light (hereinafter referred to as “infrared laser processing”) has been used for processing of a workpiece such as a metallic member, for example, micro-drilling, cutting and welding.
Infrared laser light emitted from a laser resonator used for the infrared laser processing, such as carbon dioxide laser light having a wavelength of 9.3 to 10.6 μm, can have polarization planes in a variety of directions such as a horizontal direction, a vertical direction and a direction inclined with respect to a horizontal plane by 45°. In addition, the infrared laser light can have a polarization plane shifted from a desired polarization plane. Accordingly, a polarizer has been used for a correction for shift of the polarization plane, an adjustment for intensity of the infrared laser light and a stabilization of an oscillation state of the infrared laser light.
As the polarizer, for example, there is known a wire grid polarizer in which metallic wires are arranged on the surface of a substrate made of a substrate material through which desired light transmits (for example, see Patent Literature 1). However, the wire grid polarizer is difficult to apply to infrared laser light having an intensity exceeding 10 W/cm2 (for example, continuous wave carbon dioxide laser light).
Meanwhile, as a polarizer having more excellent power resistance than the wire grid polarizer, there is known a plate polarizer in which a coated plate made of zinc selenide or germanium is arranged so as to be oriented in a direction of a Brewster's angle with respect to an incident beam, and an optical multilayer film is formed (for example, thin-film polarizer (abbreviated name: TFP) manufactured by II-VI Incorporated).